Hallo Krystian,
Your main problem here will be : can you dismount the steering wheel. Whatever you choose: recovering it or reparing it, you'll have to dismount it in order to work on the wheel. The fact this contains an Airbag system is really not helping (and I hope the electronic has plugs in order to take out the wheel...
About the leather on your actual wheel, it's used. Sweat has burned the leather and as they are made of one piece, it's quite impossible to repair it. You'll have only, in my humble opinion, the choice between covering it or replacing it completely. Depends of the size of the driver's hands. If he can still hold firmly with an added layer of leather, covering is better.
But frankly you would spend more time into trying to restaure it with very poor result, while changing it would be much "easier".
Thus